1. Field of the Invention
The present invention relates to strain sensors and in particular to multi-element strain sensors incorporating fibre Bragg gratings as the strain sensing element.
2. Description of Related Art
Strain rosettes are well known multi-element strain sensors and are widely used in mechanical testing. Strain rosettes typically comprise two or three non co-linear strain gauges mounted on a common substrate. The strain gauges are typically arranged at 45xc2x0 or 60xc2x0 to one another, to form rectangular or delta rosettes respectively.
Strain rosettes can be surface mounted or embedded inside structures and used to provide a variety of information on strain fields. For example, strain rosettes can be used to measure components of strain along and perpendicular to a principal axis, or to determine the orientation of the principal axis if it is not already known.
In the past, strain rosettes have typically employed three electrical strain gauges (e.g. resistance strain gauges) as their strain sensing elements. A known rectangular strain rosette is shown schematically in FIG. 1, and includes three resistance strain gauges S1, S2, S3 arranged at 45xc2x0 to each other. The strain gauges are mounted on a substantially planar common substrate 99 for ease of handling and to maintain their mutual orientation. For optimum results, the individual strain gauges and placed as close together as possible. Separate electrical connections are required to each sensor.
Fibre Bragg gratings are well known and can be used as temperature sensors or strain gauges as alternatives to electrical sensors, providing numerous advantages. Fibre Bragg gratings (FBGs) and their use as sensing elements are described in xe2x80x9cOptical Fibre Bragg Grating Sensors: A Candidate for Smart Structure Applicationsxe2x80x9d, Dunphy et al, Chapter 10 of Fibre Optic Smart Structures, edited by Eric Udd, 1995 John Wiley and sons, Inc., ISBN 0-471-55448-0.
A typical FBG is shown schematically in FIG. 2(a) the FBG is formed of a length of optical fibre having a core 21 surrounded by cladding material 22 having a lower refractive index than the core. The optical fibre is typically a single mode fibre (mono mode), the core diameter being sufficiently small such that for a chosen light source, light can propagate along the core in only a single mode. The single mode is guided substantially by the core/cladding boundary. The xe2x80x9clinesxe2x80x9d 11 of the grating are a series of regularly spaced perturbations of the refractive index NC of the core. The grating extends along a length L of the fibre, where L is typically in the range 1 mm to 20 mm, and the variation of core refractive index along the longitudinal axis Z of the FBG is shown in FIG. 2(b). A variety of techniques can be used to produce FBGs. In one of these techniques, the refractive index perturbations are formed in the core by masking the fibre with a phase mask and exposing it to intense ultra-violet light. In another technique, the index perturbations are formed by exposing the fibre to the interference pattern produced from two intersecting halves of a UV laser beam. The spacing X of the index perturbations is determined by the angle at which the two halves of the beam intersect. The perturbations in core refractive index produced by these techniques are typically of the order of one part in one thousand or less.
The optical fibres used to produce FBGs generally have a Protective coating outside the cladding. Before the fibre is exposed to UV light to form the grating the protective coating is removed. After exposure, the stripped portion of fibre is re-coated to restore its durability.
When a broad spectrum of light is input to the FBG as an input signal, most wavelengths pass through the grating region and form a transmitted output signal 82. However, the periodic index perturbations produce a strong Bragg reflection of components of the input signal having a wavelength xcexb, the Bragg wavelength, where:
xcexb=2XNC
Thus, a tunable detector can be used to look for a peak in the reflected signal, or a trough in the transmitted signal. The wavelength at which the peak or trough occurs therefore gives an indication of the line spacing X of the grating.
When the FBG is subjected to longitudinal strain, the spacing X changes and so results in a shift in the Bragg wavelength. To a good approximation, the Bragg wavelength is proportional to strain along the longitudinal axis. Advantageously, the grating sensor inherently tends to reject the effects of strain fields not aligned with the longitudinal axis.
Advantageously, as strain is determined by measuring the Bragg wavelength, the measurement is not affected by fluctuations in the intensity of the input light.
The fibre Bragg grating provides other advantages associated with fibre optic sensors. For example it is immune to electromagnetic interference, has light weight and small size, exhibits high temperature and radiation tolerance, and is durable even in harsh environments.
Fibre optic strain rosettes employing three separate FBGs as the strain sensing elements are known, in which each FBG has its own input and output fibres, separate from those of the other FBGS. Although the sensing region may be suitably compact (i.e. the FBGs may be arranged close together) the three sets of related fibres are inconvenient.
Rather than connecting separately to each one of an array of FBGS, it is known to instead connect them in series, provided that their nominal Bragg wavelengths are sufficiently different. One such arrangement is shown schematically in FIG. 3. Here, a light source 70 outputs a signal, a portion of which 80 is input to a series string of fibre Bragg gratings 1A, 1B, 1C via a bi-directional coupler C. The three FBGs have different nominal Bragg wavelengths, xcexBA, xcexBB, and xcexBC respectively and the reflected signal 81 returning to the coupler essentially consists of light at just these three wavelengths. A portion of the reflected signal 81 is input to a light detector 71 via the coupler C. In this example, the light source 70 is a broad band source and the light detector 71 is a tunable narrow band detector. Thus, as the detector scans across a range of wavelengths, intensity peaks will be detected corresponding to the three Bragg wavelengths, and so the strain experienced by each FBG can be determined. Thus, in FIG. 3 the fibre Bragg gratings are multiplexed.
It will be appreciated that in alternative arrangements a tunable narrow band light source may be used in conjunction with a broader band light detector to measure the Bragg wavelengths.
Strain rosettes incorporating series connected fibre Bragg gratings are known and an example is shown schematically in FIG. 4. Here, the optical fibre components of the strain rosette are formed from a single continuous fibre comprising an input portion 50 connected to a first fibre Bragg grating 1A. The first fibre Bragg grating is connected by a connecting loop 6 to the second FBG 1B which in turn is connected by a second loop 6 to a third FBG 1C. The FBGs are arranged at 0xc2x0, 45xc2x0, and 90xc2x0 with respect to a nominal axis and the rosette is encapsulated in a thin film of encapsulating material 9. The thickness of the optical fibre is exaggerated in the figure for clarity.
The three FBGs are arranged close together, forming a compact sensing portion, but the overall size of the rosette is significantly larger as a result of the connecting lengths of fibre 6 formed into loops. Although it is desirable to make the loops as small as possible to minimise the overall size of the rosette, the minimum bend radius must be large enough to avoid significant bend loss. For typical optical fibres having a cladding diameter of up to 200 xcexcm the minimum bend radius without loss is approximately 1 cm. Thus, this large minimum bend radius of the fibres results in a large and cumbersome device when the multiplexed FBG sensors are arranged in the necessary geometry.
Delta rosettes formed of multiplexed FBGs are also known, and are described for example in xe2x80x9cState of Strain Evaluation With Fibre Bragg Grating Rosettesxe2x80x9d S. Magne et al, Applied Optics, Dec. 20th, 1997, Volume 36, No.36, PP9437-9447. An example of one of these delta rosettes is shown schematically in FIG. 5. The three FBGs, 1A, 1B, 1C are connected in series by two loops L formed in the length of connecting fibre 6, and the FBGs are arranged at 60xc2x0 to one another. Again, the minimum size of the connecting loops L is determined by the minimum bend radius of the fibre, and results in the rosette having a size of approximately 8 cms.
Thus, the mutual arrangement of the FBG""s is dictated by the rosette design and function, and the connecting portions of the fibre 6 have to be appropriately routed from the end of one FBG to the xe2x80x9cinputxe2x80x9d end of another. The fact that the connecting portion of optical fibre cannot be bent over a radius of less than 1 cm without introducing appreciable loss has in the past been a serious constraint on the arrangement of the connecting portions, and has in turn meant that it has been impossible to produce practical FBG strain rosettes smaller than a few centimetres square.
It is an object of the present invention to produce smaller multi-element strain sensors, including FBG strain rosettes, especially for applications where the sensor is to be embedded in a structure. For example, a smaller device may be incorporated without compromising the strength of the structure.
According to a first aspect of the present invention there is provided a strain sensor comprising:
first and second fibre Bragg gratings, and a length of optical fibre connecting the gratings in series, each grating being substantially straight and the two gratings being non-parallel and having different nominal Bragg wavelengths, the connecting length of optical fibre having a bend,
characterised in that a portion of the connecting length is tapered, the tapered portion including an elongate waist portion having a reduced cross-sectional area, and
said bend is formed in the waist portion.
Thus, the FBG""s are multiplexed, and being substantially straight are responsive only to components of strain along their respective longitudinal axes. They are arranged non parallel to respond to different components of strain, and may, for example, be arranged substantially in a common plain at an angle of 60xc2x0, 45xc2x0 or 90xc2x0 to one another as part of a strain rosette.
Unlike previous FBG strain sensors, the connecting length of optical fibre includes a tapered portion. Tapering of optical fibres is a well known process and typically involves heating a section of the fibre in a flame and then elongating or pulling the section to form a taper waist, or neck, of reduced cross-sectional area. Generally, the taper waist is elongate with a substantially uniform cross-section and the regions of the fibre over which the cross-sectional area reduction occurs are known as the taper transition portions, i.e. the tapered portion typically comprises two taper transition portions and the taper waist portion, with the taper transition portions connecting the taper waist to untapered sections of the fibre.
The present invention exploits the fact that the taper waist portion can be bent over smaller radii than the untapered fibre without loss. Thus, the routing of the connecting length of fibre from one grating to the other can be achieved substantially by means of bends formed in the taper waist portion, and the reduced minimum bend radius places less of a constraint on this routing.
In short, by employing tapered portions in the connecting length, and forming bends of radius less than 10 mm in the taper waist, the routing of the connecting length from one FBG to the next can be more direct, enabling the size of the device to be reduced.
In general, the smaller the cross-sectional area of the taper waist portion, the smaller the minimum acceptable radius of curvature and so the more direct can be the connection. However, the taper waist portion cross-section must be sufficiently large to enable the desired wavelengths of light to propagate along the device.
Sensing apparatus comprising the strain sensor may employ a light detector arranged to detect light reflected from the gratings back down an input length of fibre, or alternatively may be arranged to measure light transmitted through the gratings and output along an output length of fibre connected to an end of the second FBG.
The strain sensor may comprise additional FBG""s connected in series with the first and second, and each connecting length of fibre may incorporate a tapered section.
The minimum acceptable bend radius of the waist portion depends on the waist portions cross-section, and for taper waist diameters of for example 20 microns and smaller, the minimum bend radius may be as small as 1 mm.
The present invention allows the construction of a fibre optic strain rosette (with concomitant advantages of optical fibre sensors) that is both compact and has a minimum number of fibre leads.
Advantageously the connecting length may be formed of single mode optical fibre, and the reduced cross-sectional area of the waist may be less than half the nominal cross-sectional area of the untapered single mode fibre.
Ends of the connecting length may comprise untapered portions of the single mode fibre, as stated above, single mode (also known as mono mode) fibres comprise a core surrounded by sheath of cladding material having a lower refractive index end than the core. The core is typically circular with a sufficiently small diameter such that only the fundamental mode can propagate down the untapered fibre. This fundamental mode is guided in the untapered fibre by the core-cladding boundary. The core diameter is typically smaller than 15 microns but other sizes are also known. Reducing the cross-sectional area in the waist portion by a factor of at least two ensures that the fundamental mode can no longer practically be confined and guided by the core material-cladding material interface in the taper waist. In this situation, the fundamental mode is guided by the cladding material external boundary (typically the interface with the encapsulating or potting material or air) as it propagates down the taper waist, and the core no longer plays a role. Initially, the fundamental mode propagates along the untapered portion of the fibre guided by the fibre core. On entering the taper transition region it sees a core of gradually reducing cross-section. There comes a point where the core is too small to guide the fundamental mode, which then xe2x80x9cbreaks outxe2x80x9d, to be guided by the external boundary of the cladding, i.e. the propagating light field is now over the entire waist cross-section.
It is known that a sufficiently tapered region of an isolated single mode fibre is less prone to bend loss than the untapered fibre because the fundamental mode, previously weakly confined by the core-cladding boundary, is strongly confined in the tapered region by the cladding-air boundary. For example, in the paper xe2x80x9cMiniature High Performance Loop Reflectorxe2x80x9d, Oakley et al, Electronics Letters, Dec. 5th, 1991, Volume 27, Number 25 PP2334-2335, it is reported that a 1.5 mm diameter bend can be formed without introducing measurable loss (i.e. in this case less than 0.05 dB) in a tapered waist region of a single mode fibre, the untapered fibre having a core diameter of 10 microns, a cladding diameter of 125 microns, and a cut-off wavelength of 1250 nanometres, and the cladding diameter in the taper waist originally reported as being 30 microns. The true cladding diameter in the taper waist was in fact 15 microns, as was reported in a correction published later. In contrast, the minimum bend diameter of the untapered fibre consistent with low loss was approximately 4 cm.
It has been determined that in embodiments of the present invention, by drawing down the optical fibres sufficiently to ensure detachment of the input fundamental mode field from the input fibre core in the taper transition region, the taper waist portion can incorporate a sharp bend with negligible additional loss. Advantageously, the taper waist portion may have a diameter of less than 50 microns.
Preferably, the taper waist portion may have a xe2x80x9cdiameterxe2x80x9d of 30 microns or smaller. In general, the smaller the diameter of the taper waist the tighter the bend which can be made without introducing unacceptable loss. However, the minimum diameter is determined by the wavelength of the light that the waist is intended to guide.
Using tapered regions of single mode fibre as connecting lengths, the connecting lengths can be routed between the FBG""s by means of bends in the waist of radii of 2 mm or smaller, and hence the overall size of the device can be significantly reduced compared with prior art arrangements.
Advantageously the connecting length may be formed of optical fibre having a core surrounded by cladding material, the cladding material having a refractive index, and the tapered portion may be contained in a first body of a first medium having a refractive index lower than the cladding material refractive index, the first body directly contacting the surface of the tapered portion.
Containing the tapered portion in such a medium maintains the strong guidance of light in the waist, enabling sharp bends to be formed in it without appreciable loss. Containment also protects the waist portion from disturbances and contamination, and may provide good adhesion to a second body containing the first.
The first body may be a coating covering the surface of the tapered portion.
The first body may extend along the entire waist portion and may completely or partially cover, coat or encapsulate the taper transition portion.
Preferably, the first body is a body of clear silicone rubber. This material has a refracture index sufficiently low to maintain strong light guidance in the waist portion, and results in no extra loss in the bent waist. Silione rubber protects the waist portion(s) and does not allow significant strain to be transmitted to the bent waist(s). The flexible silicone rubber may thus prevent the strain which the sensor is monitoring from causing unacceptable increases in losses in the bent taper waist portions.
The strain sensor may be further contained (encapsulated) in a second body of a second medium, which may be the same or different from the first medium. Preferably the second body is a body of substantially rigid material, such as epoxy resin, capable of transmitting strain to the Bragg gratings.
In order to provide even stronger guidance of light in the waist portion of the connecting length, in certain embodiments of the present invention a layer or pocket of gas is trapped in an encapsulating body containing the fibre Bragg gratings and the connecting portion, the layer or pocket surrounding the taper waist portion. The pocket may extend to fully surround the tapered portion. Light is strongly guided by the waist portion cladding materialxe2x80x94gas interface and small diameter, loss-free bends can be formed. The gas may for example, be air, and/or may be at low pressure such that the pocket essentially contains a vacuum.
To facilitate the trapping of a gas pocket around the taper portion, the sensor may include a tubular sleeve, surrounding and extending along the tapered portion. The sleeve may extend to, and form a loose seal with, the FBG""s and/or untapered sections of the connecting length, and in arrangements where the device is encapsulated (potted) the sleeve may prevent the encapsulating material from contacting the taper waist portion.
In alternative embodiments, bubbles of gas are formed in the potting material around the tapered portions.
To minimise losses, the first and second FBG""s and the connecting length may be formed from a continuous single optical fibre, which may be a single mode fibre.
Preferably, the sensor comprises a body of silicone rubber (clear) which coats or encapsulates the waist portion and so is in contact with the waist portion surface, and a body of rigid material encapsulating both the silicone rubber body and the fibre Bragg gratings. Thus, the rigid material is in contact with the surface of the Bragg gratings, but is separated from the surface of the taper waist portion by the silicone rubber.
Strain applied to the sensor (i.e. to the rigid encapsulating body) can thus be transmitted to the fibre Bragg gratings, but is not transmitted to the bent waist portion(s) because of the flexible encapsulating/coating body of silicone rubber.
Preferably, the sensor comprises two rigid plates, with the FBGs and connecting length sandwiched between. Preferably, only the FBGs are bonded to the plates, the taper waist portion being unsupported. Thus, strain applied to the plates can be transmitted to the FBGs but not to the bent tapered connecting portion. Thus, increases in losses in the or each bend when strain is applied to the sensor can be avoided or at least rendered insignificant.
Preferably, the plates are in close contact with the FBGs, i.e. the separation of the plates is basically just the diameter of the FBGs. A small quantity of bonding material can then be used to bond the FBGs to the plates. Even when the plates are separated by the minimum possible distance (set by the FBG diameters) the taper waist portion, having reduced diameter, is not trapped by the plates and can be left unsupported in air to improve light guidance. Thus, the sensor may have a laminar structure.